In the field of photomechanical process, in order to cope with diversification and complexity of printed matters, a photographic material having good original reproducibility, stabilization of processing solutions or simplification of replenishing methods have been demanded.
In particular, an original for the line camera work is fabricated by pasting therein phototypesetting letters, handwritten letters, illustrations and photographs as a halftone image. As a result, a mixture of images different in the density or line width is included in the original and a process camera, a photographic material and an image formation method capable of finishing up the work in good reproduction from the original are keenly demanded. On the other hand, in plate making of a catalogue or a large-size posture, a halftone photograph is very often enlarged (texture expansion) or shrunk (texture contraction). When the print making uses an enlarged halftone photograph, the screen ruling is roughened to result in photographing of out-of-focus dots, whereas in case of shrinking, the screen ruling per inch increases more than that of the original to result in photographing of thin dots. Accordingly, in order to maintain the reproducibility of halftone gradation, an image formation method ensuring further broader latitude is being demanded.
As a system for meeting the requirement for broader latitude, a method is known where a lith-type silver halide light-sensitive material comprising at least silver chlorobromide having a silver chloride content of 50% or more is processed with a hydroquinone-based developer suppressed in the sulfite ion effective concentration to a very low level (usually, 0.1 mol/liter or less) to obtain a line original or a halftone image having a high contrast and a high blacking density so that the image area and the non-image area can be clearly distinguished. However, according to this method, due to the low sulfite ion concentration, the developer is very susceptible to air oxidation and therefore, various efforts and designs are made so as to keep the developer activity stable in continuous use thereof.
In order to overcome such instability in image formation, an image formation system capable of providing development with a processing solution good in storage stability and achieving ultra-high photographic properties is being demanded. As one example of such a system, U.S. Pat. No. 4,166,742 proposes a system for forming an ultrahigh contrast negative image having a .gamma. value exceeding 10 where a surface latent image-type silver halide photographic material having added thereto a specific acylhydrazine compound is processed with a developer containing a sulfite ion as a preservative and having a pH of from 11.0 to 12.3. This image formation system provides superior effects with respect to sharp halftone dot quality, processing stability, rapid processability and original reproducibility.
However, since the developer used in this image formation system is designed to have a relatively high pH so as to obtain a high contrast image, a defect is present that fog is readily caused. In order to suppress the generation of fog, JP-A-61-213847 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a technique for improving the image quality to an extreme extent by incorporating a redox compound capable of releasing a development inhibitor upon oxidation into the photographic material.
On the other hand, it is known to use a certain kind of merocyanine dyes as a spectral sensitizing dye of a photographic emulsion as disclosed, for example, in U.S. Pat. Nos. 3,480,439 and 3,625,698. Further, as a sensitizing dye having properties capable of giving a contrast and sharp halftone image, JP-A-55-45015 discloses a dimethinemerocyanine dye having a thiohydantoin ring substituted by a pyridyl group and JP-B-54-34532 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a dimethinemerocyanine dye having a thiohydantoin ring substituted by a phenyl group. However, the silver halide photographic material containing such a merocyanine dye is in practice low in the sensitivity and contrast and bad in storage stability or bound to such a defect that residual dye stain after development processing is readily generated. Accordingly, coming out of a spectral sensitizing dye free of such defects has been keenly demanded.